J. Phys. I France
Volume 3, Numéro 8, August 1993
Page(s) 1839 - 1859
DOI: 10.1051/jp1:1993102
J. Phys. I France 3 (1993) 1839-1859

Magnetic structure, phase transition and magnetization dynamics of pseudo-1D CoNiTAC mixed crystals

Th. Brückel1, C. Paulsen2, W. Prandl1 and L. Weiß3

1  Institut fiir Kristallographie, Charlottenstr. 33, D-7400 Tilbingen, Germany
2  CNRS-CRTBT, B.P. 166, F-38042 Grenoble Cedex 9, France
3  Hahn-Meitner Institut, Postfach 390128, D-1000 Berlin 39, Germany

(Received 14 January 1993, revised 19 March 1993, accepted 20 April 1993)

We present the results of a combined neutron scattering and magnetization study of the pseudo-one-dimensional magnetic compounds [ (CH $_{\rm {3}}$) $_{\rm {3}}$NH] Co $_{\rm {1}-x}$Ni xCl $_{\rm {3}}$ .2 H $_{\rm {2}}$O, abbreviated as CoNiTAC. For Co rich samples, we determined the 3d magnetic structure to be canted antiferromagnetic having the magnetic space group Pnm'a'. We performed Monte-Carlo simulations and could reproduce well the measured temperature dependence of the sublattice magnetization assuming an Ising type, spatially strongly anisotropic interaction. The exchange integral along the chains ( b-direction) is at least 100 times larger than the average exchange in the a- c-plane. Despite this strong one dimensional character of the exchange interactions the critical exponent $\beta$ was found to be very close to the theoretical value for a 3d Ising system (0.31). We did not discover any anomaly of the long range order component for the mixed crystals, for which a spin glass phase had been proposed by other authors on the basis of a decay of the TRM on macroscopic time scales. Instead, our polarized neutron diffraction and magnetization measurements reveal that the low temperature dynamics is connected to domain relaxation processes and not to a spin glass transition.

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